In a conventional electric power supply for a vehicle, for example, in an electrical power supply system for an automobile, in order to supply power from a battery to its electrical loads such as lamps, motors and the like, a plurality of power lines are laid out for connecting between respective electrical loads and the battery. For a recent automobile, which has an increased number of electrical loads compared to the conventional automobile, the number of power lines has increased significantly. Further, because it is necessary to provide power lines not only for connection between respective electrical loads and the battery but also for connection between switches that control the respective electrical loads and the battery, a total distance of extension of the power lines has increased substantially. Any such increases in the number of power supplies and the total distance of extension of power lines, which leads to an increase in weight of the vehicle body, will deteriorate fuel mileage.
Therefore, for example, as described in the brochure of International Publication WO 96/26570 (1996), it is proposed that by connecting a plurality of relay circuits (control modules) to a power line which is laid out in a loop, then a plurality of electrical loads are connected respectively to each of these relay circuits through which to supply power thereto.
However, the prior art disclosed in the brochure of International Publication WO 96/26570 (1996) is associated with one problem that because every control modules are connected to a single loop power line, if a fault occurs in one of the control modules, the other control modules may be affected, and in a worst case, it becomes difficult to control the engine itself, which is an essential function for the automobile.
Further, the prior art disclosed in the brochure of the International Publication WO 96/26570 (1996) is associated with a second problem as will be described below. Although when the power line is short-circuited, its electrical loads are isolated from the power line by operating its switching devices, no countermeasure, however, is taken for a case where an over-current flows through an electrical load due to failure in the electrical load itself or the like. In order to prevent the occurrence of over-current, a fuse may be used. However, in order to improve easiness of fuse exchange, it has been preferred to place a plurality of fuses in one concentrated area. In order to place the plurality of fuses in one concentrated area, it becomes necessary to provide for additional power lines to connect between the plurality of fuses that are placed in one concentrated area and their associated electrical loads, consequently failing to reduce the length of the power lines.
The prior art technologies similar to the above are disclosed in the following documents. JPA Laid-Open Nos. 9-275632, 10-4632, 10-20970, U.S. Pat. Nos. 4,652,853 and 4,739,183.